Electron tube having improved low-impedance cathode connecting means



P. GERLACH 3,44 ELECTRON TUBE HAVING IMPROVED LOW-IMPEDANCE June 10, 1969 CATHODE CONNECTING MEANS Filed April 25. 1967 I \VI'IN'IOR.

I P/seze 6554,46 M

United States Patent Int. Cl. HOlj 7/44, 13/46, 17/34 US. Cl. 31540 8 Claims ABSTRACT OF THE DISCLOSURE The cathode in the form of a tubular wire mesh (1) supported between spaced end plates (8, 9) is connected across the connector terminals (3, 4) of the tube by means of massive conductor rods (10, 13) providing a low-resistance path for the direct current serving to heat the cathode, and is also connected to one of said ter minals (4) by way of an annular set of thin strip conductors (14) providing a parallel low-impedance path for signal frequency currents. An impedance-matching ring (16) having a tapered surface (17) provides a gradual impedance transition from the impedance between cathode (1) and grid (2), to the impedance between the respective connector sleeves (11, 12).

In the design and construction of high-power electron tubes in the higher radio, video and ultra-high frequency range, it is important for the inductance of the cathode supply conductors to be kept as low as possible. This is necessary in order to reduce losses in operation and also to increase the upper cut-off frequency and extend the frequency handling capability of the tube.

Conventional tubes of the class just specified habitually use a direct-heated cathode, frequently in the form of a tubular wire mesh secured between end plates. Connected to the respective end plates are two sets of (one or more) massive conductor rods extending parallel to the axis of the tubular cathode and having their opposite ends connected to respective terminal conductors in the form of coaxial plates of different diameters extending in sealed relation through the tube envelope. The terminal conductor of the central grid comprises too a plate extending in sealed relation through the tube envelope and in surrounding coaxial arrangement with both terminal conductors of the cathode. The central grid terminal conductor and the cathode conductor of greater diameter are intended to be connected to the external input circuit which is very often made up of a coaxial line tunable by means of a plunger. A DC. voltage source is connected to the cathode terminal conductors for providing the cathode heating-current. Thus the said massive conductor rods are, in operation, traversed both by the DC. heating current and by the signal-frequency alternating currents. While such an arrangement has been widely used and accepted as satisfactory, it has been found to increase unnecessarily the ohmic component as well as the reactive component of the input impedance of the tube especially in the higher frequency ranges, of the order of a hundred to a thousand megacycles per second and over. The reasons are the following.

As higher and higher signal frequencies are involved, the signal currents are due to skin efiect, conveyed exclusively over shallower and shallower surface or skin layers of the conductor rods. Since the conductor rods are necessarily limited in number owing to structural considerations, and since the rods must be made from high melting refractory metals, e.g., molybdenum, which 3,449,625 Patented June 10, 1969 are relatively poor conductors and may not have too good a surface condition, the flow section area for the signal frequency currents becomes insufficiently large and the ohmic resistance therefore rises. For signal frequencies of the order of some hundred megacycles per second, this resistance may assume a very high value in the conventional tube constructions, with the disadvantageous results noted above. It is an object of this invention to provide a cathode connecting arrangement in which this difiiculty is entirely eliminated.

Moreover, the conductive rods connecting the cathode to its respective coaxial plates have a great inductance. In fact said rods and cathode assembly constitutes within the tube envelope an extension of the coaxial line that makes up the input circuit outside the tube. Now the radial spacing between the adjacent coaxial electrodes is made very small, so that the characteristic impedance therebetween is very low. On the other hand, the radial spacing between the adjacent coaxial connector terminals between the conductors of the exterior coaxial line must be substantially greater, this being necessary to ensure safe insulation and also for accessibility. An impedance mismatch is therefore present as between the characteristic impedance value measured across the electrodes, and the characteristic impedance value measured across the connector terminals. The mismatch is increased by the fact that between these two coaxial line sections is inserted another line section, i.e., the coaxial line made up of the conductive rods used as cathode supply conductors, said line having a characteristic impedance higher than that of the two other coaxial line sections chiefly for wide waveband amplifiers, this double discontinuity, of the characteristic impedance increases the difficulties for obtaining a constant ohmic loading of the input circuit by the electronic input resistance of the tube. It is an object of this invention to reduce the impedance mismatch present between the cathode and grid of a power tube, and their respective terminal connectors.

According to a principal feature of this invention, there is provided in a tube having a direct-heated cathode and massive conductor means connecting each end of the cathode to a related connector terminal of the tube and providing a low-resistance path for direct current therethrough to heat the cathode, thin sheet or strip conductor means connecting one end of the cathode to one of said terminals in parallel with, but distinct from, a portion of said first path and providing in turn a low-impedance path for signal-frequency current therethrough.

In this way, optimal current flow conditions are provided in the improved tube both in respect to the large (several hundred amperes) direct heating current, and in respect to the signal frequency alternating currents.

According to a further feature of the invention, the lowimpedance path defined by the sheet conductor means includes a tapered impedance matching section.

FIG. 1 is a simplified view, generally in axial cross section, of the cathode and grid assembly of a coaxialeleotrode tube constructed according to the invention; and

FIG. 2 is a similar view of a modified embodiment.

The electron tube structure partly shown in FIG. 1 includes a cathode 1 in the form of a cylindrical tubular mesh of helical wires disposed in cross relation. The tubular mesh is secured at its respective ends to a pair of supporting discs 8 and 9 which serve for the mechanical support of the cathode and for the electrical connection thereof in a current circuit as will be presently disclosed.

The tube further includes a grid structure generally designated 2, which is cylindrical and coaxially surrounds the cathode 1. It will be understood that the complete tube assembly further includes a cylindrical anode which coaxially surrounds the grid 2 and may constitute part of the envelope of the tube. The anode and related elements are not here illustrated since they are irrelevent to the present invention and may be conventional.

The electrode supporting and connecting means illustrated comprise three coaxial terminal connector plates 3, 4, 5 arranged in the stepped relationship shown, being spaced axially with respect to one another by means of ceramic rings 6 and 7, and having downbent peripheral flanges that are spaced axially and radially from one another. The lower terminal connector plates 3 and 4 are associated with the cathode and the upper, largeradius terminal connector plate 5 is associated with the grid. Specifically, the lowermost terminal connector plate 3 has a massive conductive metal rod 10 projecting from its center, the upper end of the rod being suitably secured to the central part of the upper cathode supporting disc 8. Rod 10 extends through a central opening of the lower cathode supporting disc 9 so as to be electrically insulated from it. The intermediate terminal connector disc 4 is likewise centrally apertured for the free passing through it of the connector post 10, and has a tubular connector member 11 upstanding from an intermediate region thereof. A flanged plate 115 is fitted in and secured to the upper end part of member 11 and has a central opening through which the central post 10 freely extends. Three further massive connector rods or posts 13, have their lower ends secured in equispaced relation to the transverse wall of flanged member 115, and have their upper ends secured to corrsponding points of the lower end cathode supporting disc 9.

From the upper connector member 5 there projects a tubular member 12 to which the lower end of the grid 2 is secured.

With the arrangement so far described, which is generally conventional, it will be seen that the cathode and grid electrodes are rigidly supported in their coaxial assembled relationship. Further, in the operation of the tube, the cathode terminal rings 3 and 4 are connected across a source of direct current for heating the cathode. The heating power required may be quite large, higher than 1 kilowatt, usually with a relatively low voltage and a correspondingly high current of the order of several hundred amperes. The heating current flows, e.g., from terminal ring 3 through central post 10, upper cathode supporting flange 8, tubular mesh 1, lower supporting disc 9, rods 13 and members 11511 to the other cathode terminal ring 4. The current in flowing through the wire filaments of the tubular mesh 1, which are made of highly resistive metal having thermionic emissive properties, such as thoriated tungsten, carries the wires to a high temperature of, e.g., 1700 C., whereupon electrons are emitted therefrom outwardly past the grid 2 in the usual way.

According to this invention, the cathode has further conductor means connected to it for carrying the UHF alternating signal currents that are normally present in the operation of such a tube. As shown, the signal-frequency conductor means comprise an annular series of thin strips 14 having their upper ends connected to the lower cathode supporting disc 9, preferably by being soldered to the inner surface of a peripheral flange of said disc as shown. The lower ends of the strips 14 are similarly connected around the upper end of the upstanding cathode connector sleeve 11.

The thin conductive strips 14 provide a low-impedance conductive path for signal frequency current from (or to) the cathode 1 to the terminal connector means 11-4, this path being in parallel with the DC. current path through the rods 13. In the operation of the improved tube, therefore, the cathode-heating direct current and the signal frequency current will each follow its own low-impedance path, ensuring minimum losses and maximum efliciency of tube performance especially for very high signal frequencies, as of the order of hundreds of megacycles per second.

According to a feature of the invention, the conductor strips 14 may be shaped to provide a gradual transition for the characteristic impedance of the tube from the value thereof as measured between the cathode 1 and grid 2, to the value thereof as measured between the terminal connectors of the respective electrodes. As is well known, the electrodes of a power tube must be very closely spaced for a number of reasons including the requirement to rise the transconductance and to minimize transition time. The characteristic impedance between the closely spaced cathode 1 and grid 2 regarded as a coaxial line, is correspondingly low, being given by the equation.

Characteristic impedanco=\/%z138 log where L represents inductance, C capacitance, and R and R the radii of the outer and inner coaxial electrodes, regarded as the conductors of a coaxial line. Because of the small radial spacing between the cathode and grid, this value is extremely small being of the order of only a few ohms. On the other hand, the spacing between the terminal connectors such as 4 and 5 of the cathode and grid must be considerably larger, in order to provide safe insulation against flashover, and also for reasons for accessibility. The characteristic impedance as measured at the level of said terminal connectors is, therefore, considera'bly larger than the value measured at the level of the electrodes themselves. According to the invention, as illustrated in FIG. 1, this impedance mismatch is minimized by bending the conductor strips 14 to the over-all frustoconical shape shown, whereby to provide a gradual transition between the large-radius portion adjacent the cathode support disc 9 and the smaller-radius portion adjacent the cylindrical connector member 11. Further, a conductive ring 15 is shown connected to the inner wall surface of the cylindrical grid connector member 12 opposite the cathode connector member 11, in order to reduce the effective radial spacing therebetween.

In the modification shown in FIG. 2, the general construction of the electrode assembly is similar to that shown in FIG. 1 and only the differences will be described. It will be seen that the signal frequency conductor strips 14 in this case have their upper ends secured as by welding around an annular shoulder presented by the lower cathode supporting plate 9, which in this instance is of dished construction. Further, the strips 14 are more or less straight, or very slightly cambered, and their lower ends are secured, as with suitable solder, around an annular transition member or ring 16 secured around the circumference of the cylindrical cathode connector member 11. The transition ring 16 has a downwardly tapered frusto-conical surface 17 at its lower end, which provides the desired progressive transition between the characteristic impedance values as measured at the electrode and as measured at the terminal connectors, as explained above.

The signal frequency conductor strips 14 of the invention may be made of any suitable temperature-resistant electrically conductive metal, such as tantalum, preferably having polished surfaces to reduce radiation heat losses. The strips may be, e.g., 0.2 mm. thick, and they may have any suitable width, e.g., about 1 cm. The number of strips used and their mutual spacing have any desired values, but are preferably such that the strips are almost juxtaposed around the circumference to define an almost continuous cylindrical surface. In fact, the set of conductor strips 14 may conceivably be replaced by a continuous sleeve of thin metal sheet interconnecting the cathode end plate 9 and the cylindrical connector 11, although the construction herein described as comprising a number of separate strips is preferred for a number of reasons including convenience of assembly. The bend imparted to strip 14 in the FIG. 1 construction, and the camber imparted to the strips 14 in the FIG. 2 construction, serves to permit free expansion and contraction of the strips in response to temperature variations. If desired, a swanneck or other sinuous configuration may be imparted to the strips 14 in a suitable region of their length in order to enhance this effect.

In one practical construction of electrode assembly according to FIG. 2 applied to a TV power tube of a type generally similar to that sold by the assignees under the designation TH 491, the grid connector member 12 had an inner diameter of 43 mm. at its lower, end and the cathode connector member 11 had an outer diameter of 32 mm. The transition ring 16 had an outer diameter of 37 mm. and the axial length of the transition surface 17 was about 3 mm. Said ring 16 was made of molybdenum, but various other high-melting metals, such as tungsten, tantalum and the like may be used.

What I claim is:

1. In an electron tube the combination comprising:

a direct heated cathode;

a pair of cathode connector terminals connectable with external circuitry exteriorly of the tube;

massive conductor means connecting each end of the cathode to a related one of said terminals and providing a low-resistance path for direct current therethrough for heating the cathode; and

thin sheet conductor means connecting one end of the cathode to one of said terminals as to shunt a portion of said first path and providing a low-impedance path for signal-frequency current therethrough in parallel with but distinct from said direct-current path.

2. In an electron tube the combination comprising:

a cathode in the form of a tubular wire mesh;

a tubular grid electrode coaxially surrounding and closely spaced from said cathode; coaxially arranged connector terminal means connectable with external circuitry exteriorly of the tube and including two cathode connector terminals and a grid connector terminal;

an upper and a lower supporting plates connected to the respective ends of said tubular cathode wire mesh;

two sets of conductor rods one set having an upper end connected to said upper supporting plate and a lower end connected to a first one of said cathode connector terminals and the other set having an upper end connected to said lower supporting plate and a lower end connected to the other cathode connector terminal, whereby to define a low-resistance path for direct current for heating the cathode;

thin sheet conductor means defining a generally tubular surface having its upper end connected to said lower supporting plate and its lower end connected to said other cathode connector terminal, whereby to shunt a portion of said first path and provide a low-impedance path for signal-frequency current in parallel with but distinct from said direct-current path.

3. The combination defined in claim 2, wherein said sheet conductor means comprise an annular array of strlps.

4. The combination defined in claim 3, wherein said strips are deformed in an intermediate region of their free length to allow for thermal expansion and contraction thereof.

5. The combination defined in claim 2, wherein said low-impedance path defined by the sheet conductor means includes a tapered impedance matching section.

6. The combination defined in claim 2, wherein said sheet conductor means includes a tapered section of diminishing diameter in the direction from said lower cathode supporting plate towards said other connector terminal.

7. The combination defined in claim 2, including an annular impedance matching ring member secured around an upper portion of said other connector terminal and having the lower end of said thin sheet conductor means secured around the upper portion of said ring, and a tapered impedance matching surface at the lower part of the ring member of diminishing diameter away from said lower cathode supporting plate.

8. The combination defined in claim 1 wherein said thin sheet conductor means is approximately 0.2 mm. thick.

References Cited UNITED STATES PATENTS 2,490,786 12/1949 Deutzch 313278 X 2,534,548 12/1950 Fay et a1. 313-278 X 2,542,639 2/1951 De Walt 3l3-278 X 2,683,237 7/1954 Scullin 3l3278 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

US. Cl. X.R. 

